Fe-25Cr-1N-0, 2, 5Mn mass% alloys were subjected to isothermal heat treatment, and their microstructure formation and phase transformation behavior were investigated in order to clarify the effect of Mn addition on the transformation mechanism of high nitrogen austenite. Microstructure observation for the 1073 K heat-treated specimens revealed that the Fe-25Cr-1N alloy exhibited (α+Cr₂N) lamellar eutectoid structure, while the Fe-25Cr-1N-2Mn and -5Mn alloys did finer (α’(or retained γ)+Cr₂N) lamellar structure as well as (α+Cr₂N) lamellar eutectoid structure. It was suggested that the (α’+Cr₂N) lamellar structure had been formed through γ₁→γ₂+Cr₂N cellular precipitation followed by martensitic transformation of γ₂ on cooling to ambient temperature. Nitrogen concentration in untransformed austenite in the Fe-25Cr-1N-2Mn and -5Mn alloys was continuously decreased with progressing of (γ+Cr₂N) cellular precipitation due to nitrogen long-range diffusion from untransformed austenite to (γ+Cr₂N) cellular structure. As a result of decreased nitrogen concentration in untransformed austenite, transformation mechanism switched from (γ+Cr₂N) cellular precipitation to (α+Cr₂N) eutectoid transformation.